Difference between Combinational and Sequential Circuits

The difference between combinational circuits is that it makes use of only logic gates and hence only dependent on the present combination of inputs at any given point of time. On the contrary, Sequential Circuits are a combination of logic gates and memory elements, so the output here depends on the present and the past inputs given to it. 

• Combinational and Sequential Circuits are the two most predominant circuits used in the field of digital electronics. 
• Both of these Circuits make use of Logic gates. 

Key Terms: Logic gates, Sequential Circuits, Combinational circuits, Memory elements, Boolean expression, Arithmetic Progressions 

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Combinational Circuit vs Sequential Circuit

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The combinational and sequential circuits work to produce outputs depending upon the needs and instructions of the user. These two differ from each other mainly on the basis of components used and the output generation techniques used. 

• A combinational circuit is an aggregation of only logic gates without any memory elements attached to it. 
• The retrieval of data is not possible here, because of this. 
• A sequential circuit comprises logic gates as well as memory elements. 
• These memory elements make it possible to retrieve previous output produced and hence, the data can be easily retained.
• As there is no memory element in Combinational circuits, the output generation technique is centred only on the present input signals given to it. 
• The Output produced has no connection in any way with the previous outputs generated.
•  On the other hand, in sequential circuits, the presence of memory elements also plays a crucial role in shaping up the output generated. 
• The output is the resultant of both, the present input provided and the data of previous outputs stored in the memory elements. 
• Thus, the output produced is also influenced and impacted by the previous operations.

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What is a Combinational Circuit?

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Combinational circuits are combinations of logic gates but without any memory element in them. A typical Combinational Circuit is an integration of NAND, NOR and NOT gates. It works to produce outputs only based on the inputs given in the present moment. 

Here's more on what is a Combinational Circuit

1. The output produced relies on the present input given to it.
2. It is time-independent. So, its performance is not influenced by clock signals in any way.
3. The basic building blocks of Combinational circuits are logic gates, and these are devoid of any memory element.
4. It can be operated using both, boolean and arithmetic expressions.
5. The basic building blocks are NOR, NAND and NOT gates.
6. Absence of feedback path between input and output.
7. It is primarily used for: Arithmetic and logical functions, Data transmission, Code conversion
8. It is easy to design and handle, plus the operations are carried out at very high speeds 
9. Some examples of Combinational circuits are: Half Adder, Half Subtractor, Full Adder, Full Subtractor

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What are Sequential Circuits?

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A Sequential Circuit is typically a circuit having a combination of logic gate/ combinational circuits along with a memory element. Its performance is highly variable and is affected by both the present inputs and the previous outputs. Memory elements are devices having binary data stored within them. The state of any sequential circuit can be easily denoted by the binary data stored in its memory element. Other features of sequential circuits are listed below:

1. The quality of output is an outcome of both the current input signal and the previous outputs.
2.  It is constituted by logic gates/ combinational circuits and the memory elements.
3. It is time dependent. This means that these types of circuits follow clock cycles.
4. This time Dependency gives rise to two types of sequential circuits:- Synchronous (makes use of clock pulse), Asynchronous (doesn't follow any clock pulse)
5. The elementary building block is flip-flop (used to store non binary data).
6. Presence of feedback path between input and output. 
7. At times, sequential circuits can use the feedback by utilising the previous output (current state) as control inputs for the proceeding operations.
8. In sequential circuits the previous outputs are stored in the flip-flop as the current state, which can act as Control inputs for future use.
9. Because of its ability to store data and information and can easily retrieve any previous data, Sequential Circuits finds applications in: Central unit of digital circuits, Also in memory building devices.
10. It is hard to design, handle and maintain sequential circuits. 
11. The delivery of output takes a bit longer, exhibiting slow speed.
12. A few examples of sequential circuits are: Flip-Flops, Registers, Counters

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Difference between Combinational and Sequential Circuits 

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The differences between Combinational Circuits and Sequential Circuits are tabulated below:-

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Things to Remember 

• Combinational circuits are combinations of logic gates but without any memory element in them. 
• A typical Combinational Circuit is an integration of NAND, NOR and NOT gates, etc.
• A Sequential Circuit is typically a circuit having a combination of logic gate/ combinational circuits along with a memory element. 
• Combinational circuits have only logic gates as their components.
• Combinational circuits work on boolean expressions and arithmetic expressions as well.
• Combinational circuits and Sequential Circuits are different from each other on the basis of components used and the output generation techniques used. 
• Combinational circuits are less complex and high speed. Sequential Circuits on the other hand are more complex and offer a slower speed. 

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